"How to calculate the size of ROM"

If it truly is just ROM (ie, purely Read-Only Memory), there's nothing you can do to it other than just run whatever code is in it - so there's no need to know its actual size.
If you want to be able to program it, it must be some sort of PROM - most probably Flash - so you can read the Device ID.

"How to calculate the size of ... RAM"

If you want your code to run on a number of different hardware variants, you probably don't really need to precisely calculate the RAM size - just identify whether it's a "Big RAM" or a "Small RAM" board.
Probably the best way to do this is to have some sort of "hardware identifier" on the board that the processor can read; eg, an EEPROM that gets programmed as part of the final manufacturing test.

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